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Ethylene and its coproducts are used as feedstock monomers in many petrochemical and polymer industries. Nearly all of the world's ethylene is produced by cracking hydrocarbons such as naphtha and gas oil derived from petroleum refining or ethane and propane obtained from natural gas fractionation. Ethylene and other coproducts are produced at a high temperature in the radiant coils of steam cracking furnaces. In the furnace, these radiant coils are typically assembled as a set of pipes arranged in series and/or parallel, and placed in a number of parallel rows, called lanes along the length of the firebox. These coils form the heart of the cracking furnace and their design can have a decisive effect on furnace performance. Key considerations in cracking furnace and radiant coil design include furnace capacity, type of feedstock, feedstock flexibility, cracking severity, selectivity to light olefins, run length, efficiency, emissions, and cost. In this chapter, radiant coil technologies are presented based on the specific aspect that is being improved: • surface-to-volume ratio • radiant coil arrangement • internal heat transfer coefficient How these various aspects are implemented in radiant coil designs is demonstrated using a number of examples. Some of these examples are discussed in more detail such as the Triple-lane design and the Swirl Flow Tube® (SFT®). The Triple-lane design is an example of an improved radiant coil arrangement, while the SFT® is a typical example of a tube design with an increased internal heat transfer coefficient. The Triple-lane design is a specific version of a novel, multilane radiant coil arrangement that has been developed and patented by TechnipFMC and which leads to improved performance compared to conventional layouts. The Triple-lane arrangement can be used to obtain longer furnace run length, higher capacity, and better olefins selectivity with the highest ethylene production per radiant box volume. The design may be tuned to maximize one or more of these advantages. The SFT® is a second proprietary patented technology of TechnipFMC that is also discussed in more detail in this chapter. The SFT® is a small amplitude helical pipe that can be a part of cracking furnace radiant coils to enhance internal heat transfer. The stimulus for the SFT® development was the desire to enhance heat transfer. Cracking furnace licensors until now have maximized heat transfer rates by designing radiant coils to operate close to the maximum allowable metal temperatures at the end of the run. Further increase in heat transfer rates are limited by the metallurgy of the radiant coils. The SFT® technology overcomes this barrier by shifting the focus from wall temperatures to the heat transfer mechanism. The swirl flow enhances the heat transfer from pipe wall to process gas by achieving a more intense gas mixing at the tube circumference compared to conventional straight concentric tubes. This is at the expense of a slightly higher pressure drop. Using the SFT® as an example, this chapter demonstrates how a structured development approach based on the so-called technology readiness levels (TRL) can be adopted to develop the technology in various small steps from the conceptual phase to a full industrial application. The heat transfer enhancement of the SFT® technology was demonstrated in a series of experiments starting with computational fluid dynamics (CFD) studies to determine theoretically the benefits of the technology. Next, the cold flow performance was determined and compared with the CFD results followed by performance tests under real cracking conditions using the pilot steam cracking furnace of the University of Gent. The results revealed that SFT® technology can be used to enhance steam cracking furnace performance in several ways: increased run length, higher capacity, better selectivity, or a combination thereof. It is shown that with the application of this technology, a capacity increase in the range of 10%–20% can be achieved for various diverse feedstocks. Even though the base case designs used in these examples (for the purpose of comparison) are TechnipFMC's conventional radiant coils, the conclusions that are drawn are just as valid for other conventional coil layouts, including those of competitors. Lastly, all the presented new technologies can be applied in new furnaces as well as in furnace improvement/expansion projects. |
